Nutrient Limitation of Decomposition in Hawaiian Forests

In Hawaiian montane forests, we assessed whether the same nutrients limit decomposition and aboveground net primary production (ANPP) along a soil chronosequence where nutrients demonstrably and predictably limit ANPP. At three sites that vary in parent material age (300, 20 000, and 4.1 × 106 yr), we used fertilization to assess whether nitrogen (N) and/or phosphorus (P) limit decomposition. Reciprocal transplants using litter bags allowed us to distinguish limitation by externally supplied nutrients vs. limitation by nutrients within litter. Nutrient limitation of decomposition was not predictable from nutrient limitation of ANPP, in that elevated litter and soil N had only small, if any, effects on decomposition, even at the young site where N limits ANPP. At the oldest site where P limits ANPP, both elevated litter P and increased availability of soil N and P increased decomposition rates. Thus, nutrients may limit decomposition more strongly in low-P than in low-N ecosystems. Fertilization affected litter nutrient dynamics more strongly than it did decomposition, and we observed uptake of both N and P by decomposers that was not always accompanied by changes in decomposition rates. Such nutrient incorporation into decomposing litter may retain nutrients within ecosystems, even when nutrients do not limit decomposition rates.